We are living in a gaseous world and the type of gases surrounding our everyday life, for example in where we live, work or play, is vital to our well-being, safety, and even our very survival. Exposure to prolonged insufficient oxygen levels (˜15% or less) can make us very sick or might even be fatal to us at times. Too much water vapor in the air surrounding us, especially when the temperature is very high (>90° F.), can make us very uncomfortable or seriously ill. For older folks, exposure to high humidity and very high temperature for prolonged periods of time can even be fatal. Unchecked exposure to, or unintentional breathing of, toxic gases above a certain high concentration level such as Carbon Monoxide (70-400 ppm), Hydrogen Sulfide (50-200 ppm), Formaldehyde (>50 ppb) etc., to name just a few, is extremely hazardous to one's health and often leads to unexpected deaths.
In order to prevent accidental or unintended exposure to unsafe levels of gases, humans have long devised, literally from centuries ago until today, various means of detecting all manners of gases, whether they are actually harmful to them or not. Today one can classify all the significant and still prevalent gas measurement techniques developed to date into two broad categories, namely, interactive and non-interactive types. Among the interactive types are electrochemical fuel cells, tin oxide (SnO2) sensors, metal oxide semiconductor (MOS) sensors, catalytic (platinum bead) sensors, photo-ionization detectors (PID), flame-ionization detectors (FID), thermal conductivity sensors etc., almost all of which suffer from long-term output drifts, short life span and non-specificity problems. Non-interactive types include Non-Dispersive Infrared (NDIR), photo-acoustic and tunable diode laser absorption spectroscopy (TDLAS) gas sensors. Up and coming non-interactive techniques advanced only during the past two decades include the use of the latest micro electromechanical technologies such as MicroElectronic Mechanical Systems (MEMS) and the so-called Nanotechnology. However, probably a few more years have to pass before the potential of these new non-interactive type gas sensors is fully obtainable.
With so many gas detection techniques available over the years, one could easily be misled to believe that gas sensors today must be plentiful and readily available to people to avoid harmful exposure to unhealthy or toxic gases. Unfortunately, at the present time, this is far from being the truth. The reasons are constraints arising from sensor performance and sensor cost. As a result, gas sensors today are deployed for safety reasons only in the most critical and needed circumstances. An example can be cited in the case of the kerosene heater. A kerosene heater is a very cost effective and reliable appliance used all over the world for generating needed heat during the winter months. However, it can also be a deadly appliance when used in a space where there is inadequate ventilation. In such a situation, as oxygen is being consumed without adequate replenishment, the oxygen level in the space can drop to a point (<15 volume %) where it is injurious or even deadly to inhabitants if they are not adequately forewarned. Therefore, by law or code most every worldwide locales where kerosene heaters are used, this appliance must be equipped with a low oxygen level alarm sensor. Unfortunately, the lowest unit cost for such a sensor available today is only of the electrochemical type. Even so, the unit cost is still in the range of US$15-20. Furthermore, such a sensor is not even stable over time and has a life span of only 3-5 years, far shorter than the 15-20 years expected for the kerosene heater.
In short, gas sensors available to the public today for use to guard against accidental or unintended exposure to unhealthy or toxic gases are very limited and are invariably inadequate taking into consideration both performance and unit sensor cost. This situation will continue to prevail if no breakthrough gas sensor technology is forthcoming.
Although the Non-Dispersive Infrared (“NDIR”) technique has long been considered as one of the best methods for gas measurement, at least from the performance standpoint as being highly specific, sensitive, relatively stable, reliable and easy to maintain and service, it still falls far short of the list of sensor features optimally or ideally needed today. This list of the most desirable gas sensor features will be briefly described below.
The first and foremost desirable feature of a gas sensor to be used for alerting people when they are faced with harmful or toxic gases exceeding a level limit is output stability over time or what is sometimes referred to as having a thermostat-like performance feature. This feature reflects, in essence, the reliability or trust in the use of the sensor. The experience of most people in the use of a thermostat at home is that they are never required, once the sensor is installed, to re-calibrate the sensor and its output stays accurate over time. Such is not the case for gas sensors at the present time. As a matter of fact, no gas sensor today has this desirable feature of having its output stay drift-free irrespective of any measurement technology used for its design and construction.
Gas sensors today have to rely upon periodic re-calibration or output software correction in order to be able to stay drift-free over time. Most recently, the present inventor advanced in U.S. patent application Ser. No. 12/759,603 a new NDIR gas sensing methodology which renders to first order the output of an NDIR gas sensor designed using this methodology virtually drift-free over time without the need for any sensor output correction software or periodic re-calibration. Thus, it appears hope now exists for the first time for achieving the first and foremost desirable feature of a gas sensor.
The next most desirable feature of a gas sensor is its sensitivity accuracy or its ability to accurately detect the gas of interest to a certain concentration level (e.g., so many ppb or ppm), even in a temperature or pressure hostile environment. Closely related to this feature is detection specificity, namely the capability of a gas sensor to detect the gas of interest free from any interference by other gases in the atmosphere. Another desirable feature of a gas sensor is its ruggedness or its ability to withstand reasonable mechanical abuse (such as a drop from a height of 4-5 feet onto a hard vinyl floor) without falling apart or becoming inoperable. A further desirable feature of a gas sensor is its size and weight, since it is generally desired that such a sensor be small and as light-weight as possible. Yet another desirable feature of a gas sensor is its operating life expectancy (and it is desirable that it have a life span of 15-20 years, or more). Last, but certainly not least, it is desirable that the unit cost of a gas sensor be low enough that it can be affordably applied anywhere. Other than sensor output stability over time, a low unit cost feature is by far the most important desirable feature of a gas sensor, but is also the most difficult to overcome.
It is amply clear that none of the gas sensors available for purchase and use by the general public today meet all of the desirable performance and low unit cost features outlined above. Nevertheless, the long-felt need to have such gas sensors available has not diminished one single iota. The object of the current invention is to advance a novel design for NDIR gas sensors, building upon U.S. patent application Ser. No. 12/759,603 by the present inventor, such that all the desirable features in sensor performance and sensor unit production cost, hitherto unavailable to the general public, can be attained.
The novel design of the present invention can be modified to increase its sensitivity. When this is done, the new design is especially well suited for applications requiring an intrinsically safe design. One such application is in the field of mining.
Coal and crude oil are two of the most important fossil fuels in use in the world today to satisfy our energy needs. Particularly in countries like the U.S. and China, where there are enormous deposits of coal in their land, mining of coal is even more important, if not indispensable. No doubt the acquisition of other energy sources such as gas and crude oil also involves dangerous everyday operations, but coal mining has to take the top spot as far as the number of workers that perish every year is concerned. It is believed that explosions in mines alone inside China have claimed more than half a million lives during the past decade. Although the number of miners killed elsewhere in the world during mining operations is far less than those reported inside China, the number still runs into many thousands every year.
The cause of explosions inside mines has become fairly well understood over the years. The presence of methane gas (CH4) pockets is known to exist and scatter unpredictably among rocks containing coal deposits. Methane gas is odorless and the lower explosion limit (LEL) of methane gas is around 5.0 volume percent in air containing ˜21 vol. % of oxygen. It is generally believed that underground mine explosions are caused by miners accidentally and unknowingly hitting a methane gas (CH4) pocket in the mine while they are crushing and churning rocks by hand or with massive machines to get to coal deposits in tunnels. Without knowing the existence of an explosive air mixture in their work area so as to stop working immediately, the miners' operation continues to generate sparks that ultimately lead to the unfortunate explosion. Such underground mine explosions could surely be prevented if only the miners knew that immediate ambient air they are breathing has reached a lower explosion limit (LEL) for methane gas and they have to immediately stop operating their machines or rock churning by hand in order not to generate any sparks that could set off an explosion. Although methane gas sensors can detect LEL concentration levels for methane gas when such sensors are stationed at adequate distances inside mine tunnels, it is not always the case that such a sensor is in the immediate vicinity of the space where the miners are doing the heavy work. Without the presence of such a methane sensor in the space to warn the miners of such a dangerous situation where they work, underground mine explosions will inevitably occur from time to time causing the lives of many miners every year.
It has long been understood and believed that in order to eliminate the danger of underground mine explosions caused by the methane gas, one has to fulfill two important monitoring functions for mines. The first is an integrated communication and tracking system designed specifically for use in underground mines. Such a system not only is able to continuously track the exact whereabouts of the miners underground, it is also capable of monitoring in real time the outputs of all the installed gas sensors stationed inside the mine in order to be able to assess at all times any dangerous levels of gas built-ups at locations that might trigger an explosion. Over the past decade a small number of such integrated communication and tracking systems have become available. Within the last couple of years, some of them have even been installed for testing in a small number of mines around the world. For tracking individual miners working underground, an effective way is to install wireless location sensors in the helmets of miners that communicate directly with the central system above ground. The whereabouts of individual miners underground can now be continuously tracked and notified if necessary to evacuate from specific locations in case of potential danger.
But while the availability of such an integrated communication and tracking system for mines is a necessary requirement for eliminating the danger of underground mine explosions, it is not sufficient by itself to eliminate such danger. The reason is relatively straightforward. Although an expertly functioning communication and tracking system can pin point the location of a potentially explosive environment via monitoring of a fixed system of methane sensors strategically scattered throughout the tunnels of the mine, it cannot follow the exact locale of a crew of miners underground at work. If the crew cannot sense the danger of an explosive environment they find themselves in while they are working, an explosion can still occur. However, if the crew is provided with means to accurately and reliably detect the dangerous level of methane in their midst, they can immediately take action to avoid the possibility of explosions and evacuate the site. Meanwhile the central system can also take note of the dangerous condition at this location and notify other miners nearby to evacuate until the environment is under control and is safe again.
The ability of an integrated communication and tracking system for mines to pin point the whereabouts of every miner working underground can be achieved via installation of a wireless location sensor in the helmet of each of these miners. Imagine that the helmet of every miner working underground is also equipped with a wireless and intrinsically safe methane sensor capable of accurately detecting a dangerous level of methane (like the LEL) in the vicinity of working miners; in this scenario, the second important monitoring function necessary and sufficient to eliminate the danger of underground mine explosions mentioned will be fulfilled.
However, despite a long felt need for increased mine safety, and the imperative of saving miner's lives, an integrated communication and tracking system for mines does not yet exist, at least not with a methane sensor that can adequately function in such a system. This invention fulfills this long felt need by providing an intrinsically safe methane sensor that satisfies the criteria necessary for a methane sensor to be effectively and economically integrated into a communication and tracking system for mines.
This invention also provides an intrinsically safe NDIR gas sensor in a can that is an improvement over my earlier disclosed inventions acknowledged above in cross reference to related applications.